added AspNetMqttServerOptionsBuilder

Source/MQTTnet.AspnetCore/AspNetMqttServerOptionsBuilder.cs

@@ -0,0 +1,15 @@
+using MQTTnet.Server;
+using System;
+
+namespace MQTTnet.AspNetCore
+{
+    public class AspNetMqttServerOptionsBuilder : MqttServerOptionsBuilder
+    {
+        public AspNetMqttServerOptionsBuilder(IServiceProvider serviceProvider)
+        {
+            ServiceProvider = serviceProvider;
+        }
+
+        public IServiceProvider ServiceProvider { get; }
+    }
+}

Source/MQTTnet.AspnetCore/ServiceCollectionExtensions.cs

@@ -21,13 +21,13 @@ namespace MQTTnet.AspNetCore
             return services;
         }
         
-        public static IServiceCollection AddHostedMqttServer(this IServiceCollection services, Action<MqttServerOptionsBuilder> configure)
-        {
-            var builder = new MqttServerOptionsBuilder();
-
-            configure(builder);
-            
-            services.AddSingleton<IMqttServerOptions>(builder.Build());
+        public static IServiceCollection AddHostedMqttServer(this IServiceCollection services, Action<AspNetMqttServerOptionsBuilder> configure)
+        {            
+            services.AddSingleton<IMqttServerOptions>(s => {
+                var builder = new AspNetMqttServerOptionsBuilder(s);
+                configure(builder);
+                return builder.Build();
+            });
 
             services.AddHostedMqttServer();